When Giaccio was about three-quarters of the way through trimming the 50-by-100-foot area with a red-and-black power mower, Mr. Trump made a special appearance. Frank briefly acknowledged the president and greeted him but continued his work.

"Future of the country right there. We're lucky. That's the real future of the country," the president said. "Maybe he'll be president."

Mr. Trump gave Giaccio several high-fives and pats on the back as he continued cutting. He rooted him on and perused his line work on the tall, green fescue lawn.

"Have you done this before?" the president joked as he followed Giaccio.

During a brief mowing pause, the president asked him what he wanted to be when he grew up.

"This is Frank. He is going to be very famous," Mr. Trump said. "He is going to be a Navy Seal one day. He's going to do great things for our country."

A U.S. Park Service employee and Giaccio's father supervised his work, and afterward, Mr. Trump invited them in for a behind-the-scenes tour of the Oval Office and to snap pictures commemorating the occasion.

White House press secretary Sarah Huckabee Sanders had talked about Giaccio's letter during a late-July press conference. Sanders applauded Giaccio's knack for combining his landscaping skills with business savvy, and she promised that the president would allow him into the White House one day. Giaccio wrote the letter to the president when he was 10 years old, but has since had a birthday.

His letter read: "Dear Mr. President, It would be my honor to mow the White House lawn for some weekend for you. Even though I'm only 10, I'd like to show the nation what young people like me are ready for. I admire your business background and have started my own business. I've been mowing my neighbor's lawn for some time. Please see the attached flyer. Here's a list of what I have, and you're free to pick whatever you want: power mower, push mower, and weed wacker. I can bring extra fuel for the power mower and charge batteries for the weed wacker. I will do this at no charge. Sincerely, Frank."

Giaccio concluded his freelance landscaping gig at the White House with a stop at the press briefing room. There, he posed for photographs and thanked the president and Sanders for inviting him. Afterward, Giaccio called it "probably the biggest day" of his entire life and promised he won't charge his neighbors more just because he can now add "White House lawn cutter" to his resume.